In some application, such as in Electronic Intelligence (Elint), aircraft traffic management, radio navigation and distress transmitter location, a radio receiver (e.g., located at the control tower in aircraft traffic management systems) may receive a plurality of different radio signals originating from different radio emitters (e.g., the airplanes). To facilitate efficient processing of these received radio signals, it may be required to classify or sort the received signals according to certain parameters. These parameters are, for example, frequency, Pulse Repetition Interval (PRI), Pulse Width (PW), Time Of Arrival (TOA), Direction Of Arrival (DOA) and power.
Prior to classifying the received radio signals according to the DOA, the DOA of the radio signals should be determined. Systems for determining the DOA a radio signal emitted by a radio emitter are known in the art. A radio emitter is referred to herein as a device, which generates (i.e., a source) or reflects electromagnetic radiation at a frequency range extending substantially within the radio frequency band (e.g., between 30 kilo Hertz and 300 Giga Hertz). Electromagnetic radiation emitted by a radio emitter is also referred to herein as a radio signal. A known in the art technique used to determine the DOA of a radio signal is to determine the phase difference of a radio signal received by at least two antennas. In the two-dimensional (2D) case, a radio DOA determining system employs at least two antennas and measures the difference in the phase of the radio signal received by one antenna, relative to the radio signal received by the other antenna. The difference in phase between the radio signal received by the two antennas, relates to the DOA of the radio signal (i.e., the direction at which the radio emitter is located) as follows:Δφ=(2πd/λ)sin θ  (1)where Δφ represents the difference in phase between the radio signal received by the two antennas, ad represents the relative distance between the two antennas, λ represents the wavelength of the radio signal and θ represents the DOA of the radio signal. Equation (1) has a single solution as long as the relative distance d, between the two antennas is smaller or equal to half the wavelength of the radio signal (i.e., d≦λ/2).
In general, receiving radio signals via two antennas requires two receiving channels (i.e., either two separate synchronized receivers or a dual channel receiver). However, the two antennas may alternately be coupled with a single channel receiver via a switch.
U.S. Pat. No. 5,457,466 to Rose, directs to a method for employing linear arrays for measuring the angle of arrival of a signal to determine the direction (i.e. azimuth and elevation) of an emitter from an aircraft. The linear arrays may be mounted on a single or multiple aircrafts. When two or more platforms are used, monopulse phase measurements from each platform are employed to determine the azimuth and elevation of the emitter. The measurements from the multiple platforms require no time-simultaneity but rather the locations and attitudes of the platforms. However, the origin of the phase measurements, (i.e. whether single platform or multiplatform), is immaterial to the described method. According to the method directed to by Rose, a virtual spatial array is generated from the linear arrays based on aircraft six-degree-of-freedom (6DOF) motion. The baselines at different times are assumed to generate AOA cones all having a common origin, the intersection of these cones gives the emitter DOA.
U.S. Pat. No. 5,334,984 to Akaba directs to a direction finding system and method for locating the direction of a source from a receiver. According to one embodiment directed to by Akaba the receiving station is equipped with a change-over switch disposed between direction-finding receiver antennas. As the direction-finding receiver antennas are switched by the switch and the amplitude of each antenna is measured. A signal representing the direction information is determined by averaging the received signals from each antenna. According to one embodiment directed to by Akaba, an average of a signal S, representing direction information, is determined. In another embodiment, averaging is performed for each and every received signal Ea and Eb, from which the signal S is determined.
U.S. Pat. No. 6,255,911 to Hedin directs to an interferometer antenna system for measuring the angle of arrival of RF signals. The system directed to by Hedin includes a plurality of antennas connected to switching network. The switching network is connected to only two interferometer receivers. The switching network selectively connects the plurality of antenna elements in pairs to the two receivers in a sequential manner. Thus, the phase difference is measured between the received signal for several of the pairs of the antennas. Additionally, a plurality of guard antennas are coupled to a second switching network. A single guard receiver is coupled to the second switching network. The second switching network selectively connects one of the guard antenna elements to the guard receiver in a sequential manner. A controller employs a suitable angle of arrival algorithm to generate a best estimate of the direction of the received signal based on the phase signals from the two receivers and the power signal from the guard receiver.
U.S. Pat. No. 2,860,336 to Earp et al, entitled “Radio Direction Finding System” directs to a short wave direction-finding station, which includes an array of nine antennas uniformly spaced around a circumference of a circle and a reference antenna located sufficiently far from the array to ensure that negligible interference shall occur between the antennas. The antennas in the array are commutatively coupled with one channel of a two-channel radio receiver. The reference antenna is coupled with a second channel of the two-channel radio receiver. The signal from the two radio channels are mixed together to produce a signal exhibiting a phase which varies in accordance with the instantaneous difference between the phase of the signal received by the commutated antenna in the array and the signal received by the reference antenna. A phase discriminator determines the phase difference between the signal received by the currently active antenna and the signal received by the antenna, which was active one commutation step earlier